SUMMARY ROLE OF CRYPTOCHROME IN DNA DAMAGE RESPONSES AND THE CIRCADIAN CLOCK : Cryptochrome is a photosensory flavoprotein and a core component of the molecular clock which regulates the circadian rhythms of many physiological functions. We have recently discovered that cryptochrome and the molecular clock regulate cellular responses to genotoxic stress including DNA repair, apoptosis, and DNA damage checkpoints. The goal of this research project is to understand how the circadian clock controls cellular responses to DNA damage and how cryptochrome carries out its light-dependent and light-independent functions in the animal circadian clock. To accomplish these two goals we will perform the following experiments. Aim 1: Circadian Regulation of Cellular Responses to DNA damage. a) Regulation of Nucleotide Excision Repair. We recently discovered that excision repair exhibits high amplitude circadian oscillation in mice and humans. We will use genetic and biochemical approaches to understand the regulatory mechanism and establish a rational basis for chronochemotherapy. b) Regulation of Apoptosis by Cryptochrome. We have found that inactivation of Cryptochrome in p53 null mice derepresses an apoptotic pathway. We will solve the mechanism of apoptosis reactivation and investigate its potential use in cancer chemotherapy. c) Regulation of DNA Checkpoints. We have found that the UV damage-initiated checkpoint response is regulated by the clock. We will investigate the molecular basis of this connection. Aim2: Mechanism of the Action of Cryptochrome in the Circadian Clock. a) Repressor Function. We will purify mammalian cryptochrome and other clock proteins and determine the mechanism by which cryptochrome inhibits the Clock-BMal1 activator in an in vitro system. b) Photosensory Function. We will conduct photochemical/photophysical experiments to elucidate the photosensory function of cryptochrome.